This invention relates to brake systems and brake system components and, more particularly, to vehicular brake systems such as for bicycles or the like.
In general, the invention involves the use of a braking material providing a braking surface having the characteristic of maintaining a substantially uniform braking force regardless of the presence of liquids, such as water, or lubricants, such as oil or grease or the like, on the braking surface or between braking surfaces.
The coefficient of friction characteristics of conventional brake materials are well known and it is a well known fact that conventional brake materials have substantially different coefficients of friction under dry and wet operating conditions. For example, conventional rubber compound braking materials, such as commonly used for bicycle brakes, have a dry coefficient of friction (cf) of approximately between 0.5 and 0.9 and a wet cf of approximately between 0.1 and 0.03. All references herein to coefficients of friction are based upon standard measurements of the material against steel. Thus, under wet operating conditions, conventional bicycle brakes have very little braking capability and the available braking force may vary widely resulting in substantial danger to a bike rider. Indeed, bicycle accident statistics show that bicycle brake problems are a leading cause of bicycle accidents. Of course, loss of braking capability is a problem in any braking system for any use and wet braking problems have been long known in vehicular braking systems for airplanes, automobiles and trucks.
It has been conventional practice in the brake art to utilize materials for brake pads which have relatively high dry cf and may be generally categorized as "friction materials" as opposed to bearing type materials known in the bearing art as "anti-friction materials". While it has been conventional practice in the brake art to attempt to solve braking problems and to provide better braking systems by utilization of "friction materials" having relatively high coefficients of friction, in general, the present invention involves the use of "anti-friction materials" which have relatively low coefficients of friction. In addition, while it has been conventional practice to attempt to solve set braking problems by elimination of the cause of wet conditions and/or by temporarily effecting changes in a braking system designed for normal operation under dry conditions, a presently preferred embodiment of the invention involves a brake system utilizing an "anti-friction" brake pad material which normally operates in a "wet" relatively low of condition so as not to be affected by changes in coefficient of friction due to variations in wet and dry operating conditions.
An important object and result of the present invention is the provision of a brake system utilizing an "anti-friction" brake material having substantially the same coefficient of friction in all conditions of operation so that there is no substantial difference in operation of the system in wet or dry conditions.
In the presently preferred form of the invention, the material used is Oilite which is generally known and widely used as a low friction bearing material. Oilite material may be characterized as a relatively hard, non-compressible by applied braking forces, porous (i.e., permeable by liquids), sintered metal powder material, such as bronze, which may be impregnated or coated with a lubricating material such as oil or grease. Such a material has a coefficient of friction of 0.34 when dry and without lubricant and a coefficient of friction of 0.17 when lubricated or wet.
I have discovered that the use of such a material for a brake substantially eliminates the problem of brake "fade" in wet operating conditions. Brake "fade" reduction in coefficient of friction results not only from wet conditions but also from increases in temperature in use. The use of low friction metallic bearing materials has the additional advantage of providing higher more rapid heat transfer than conventional friction type brake materials so as to reduce brake "fade" problems resulting from excessive heat. In addition, such low friction metallic bearing materials have relatively high plastic flow points, e.g., approximately 600.degree. F. to 850.degree. F., as compared with conventional high friction brake materials such as rubber compounds which have substantially lower (e.g., approximately 200.degree. F. to 300.degree. F.) plastic flow points whereat the surface characteristics are subject to being permanently altered. Thus, the low friction metallic bearing materials are substantially less subject to heat related deterioration that other high friction brake materials.
In order to achieve required levels of braking force by use of an "anti-friction material" providing a relatively low coefficient of friction, the present invention further comprises new and improved apparatus and methods for applying relatively high braking forces sufficient to achieve desired braking results without danger of brake locking. Such apparatus and methods may also provide for self powering and/or self energizing of the brake by utilizing the energy of the member whose motion is to be changed to increase the applied braking force.